DNS of separating, low Reynolds number flow in a turbine cascade with incoming wakes

Abstract

Three-dimensional direct numerical simulations (DNS) of flow in a low-pressure turbine cascade at high angle of attack have been performed. The large angle of attack is found to cause separation at the leading edge and somewhat upstream of the trailing edge along the suction side of the blade. The separation bubble at the leading edge is small and any disturbances generated are damped downstream by the accelerating flow due to the favourable pressure gradient. The boundary layer at the downstream half of the suction side separates as soon as the impingement of free-stream disturbances, stemming from a passing wake, ends. The separated boundary layer is very unstable. After some transient time, small disturbances cause it to roll up. Shortly after free-stream disturbances impinge again, it completely vanishes. Near the trailing edge the flow remains turbulent at all times. As in the simulations performed by Wu and Durbin [J. Fluid Mech. 446 (2001) 199–228] longitudinal vortical structures along the downstream half of the pressure side of the blade are obtained by straining of passing wakes. Along the upstream half of the suction side similar longitudinal vortices are found to be formed by severe stretching of the wake by the strong main flow. Because of this stretching, these vortices are almost aligned with the surface of the blade as they impinge on the boundary layer. In contrast to the vortical structures found at the pressure side, the suction side vortical structures only exist for a short time.